The invention relates to separations conducted by pressure swing adsorption (PSA). The present invention provides simplified controls, with enhanced flexibility of control adjustment through flow regulation under changing operating conditions.
Gas separation by pressure swing adsorption is achieved by coordinated pressure cycling and flow reversals over adsorbent beds which preferentially adsorb a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure is elevated to a higher pressure during intervals of flow in a first direction through the adsorbent bed, and is reduced to a lower pressure during intervals of flow in the reverse direction. As the cycle is repeated, the less readily adsorbed or xe2x80x9clightxe2x80x9d component is concentrated in the first direction, while the more readily adsorbed or xe2x80x9cheavyxe2x80x9d component is concentrated in the reverse direction.
The conventional process for gas separation by pressure swing adsorption uses two or more adsorbent beds in parallel, with directional valves at each end of each adsorbent bed to connect the beds in alternating sequence to pressure sources and sinks, thus establishing the changes of working pressure and flow direction. Valves are required to control feed gas admission and discharge of gas enriched in the heavy component at the feed ends of the adsorbent beds, to control delivery of gas enriched in the light component at the product ends of the adsorbent beds, and to control depressurization and repressurization steps from either the feed or product ends of the beds.
Enhanced separation performance is achieved in well known PSA cycles using steps for each adsorbent bed of cocurrent feed at the higher cycle pressure, cocurrent initial blowdown, countercurrent final blowdown, countercurrent purge at the lower cycle pressure, and countercurrent pressurization. As disclosed by Kiyonaga (U.S. Pat. No. 3,176,444), Wagner (U.S. Pat. No. 3,430,418) and Fuderer et al (U.S. Pat. No. 3,986,849), improved product recovery can be obtained with more than two adsorbent beds operating in parallel, by performing pressure equalization steps between the separate beds so that a first bed undergoing a pressure reduction step exchanges gas which typically has been substantially purified to a second bed undergoing a pressure increase step so that the working pressure of the first and second beds is equalized to a pressure intermediate between the high and low pressures of the cycle.
With a greater number of beds, multiple pressure equalization steps can be achieved, although the valve logic and controls are then greatly complicated. Modern industrial scale PSA plants with six or more beds (e.g. as described by Fuderer et al for hydrogen purification) use a large number of two-way valves under computer control to establish both the cycle switching logic and adaptive flow control of each step.
It is well known that the complexity of valving in PSA systems may be reduced by use of multiport valves to establish the cycle switching logic. Thus, Snyder (U.S. Pat. No. 4,272,265) has disclosed a rotary distributor valve for controlling high pressure feed and low pressure exhaust flows for an air separation pressure swing adsorption system with multiple beds. Use of a coaxially aligned pair of distributor valves, respectively controlling feed and product gas flows at opposite ends of the beds, was disclosed by van Weenen (U.S. Pat. No. 4,469,494), Hill (U.S. Pat. No. 5,112,367) and Hill et al (U.S. Pat. Nos. 5,268,021 and 5,366,541) have disclosed oxygen concentration PSA devices using multiport rotary valves with stationary adsorbent beds. The processes disclosed by van Weenan and Hill have pressure equalization steps conducted at respectively the product or feed ends of the adsorbent beds.
Prior art PSA systems with multiport distributor valves have been used commercially in small scale oxygen enrichment applications, as recommended by Dangieri et al (U.S. Pat. No. 4,406,675) for a rapid PSA process in which flow control is intentionally established by relatively steep pressure gradients in the adsorbent bed. The adsorbent bed must therefore be spring-loaded or otherwise immobilized to prevent attritional damage.
For large industrial PSA systems, mechanical immobilization of the adsorbent beds has not been practicable. Careful flow control is required to ensure that pressure gradients in the adsorbent bed are kept low, well below the onset of fluidization.
Mattia (U.S. Pat. No. 4,452,612) and Boudet et al (U.S. Pat. No. 5,133,784) disclose PSA devices using a rotary adsorbent bed configuration. The multiple adsorbent bed ports of an adsorbent bed rotor sweep past fixed ports for feed admission, product delivery and pressure equalization; with the relative rotation of the ports providing the function of a rotary distributor valve. Related devices are disclosed by Kagimoto et al (U.S. Pat. No. 5,248,325). All of these prior art devices use multiple adsorbent beds in parallel and operating sequentially on the same cycle, with multiport distributor rotary valves for controlling gas flows to, from and between the adsorbent beds.
An advantage of PSA devices with the adsorbent beds mounted on a rotary adsorbent bed assembly, as in the cited prior art inventions by Mattia and Boudet et al., is that function port connections for feed, exhaust, product and pressure equalization are made to the stator and are thus accessible to flow control devices. However, a rotary adsorbent bed assembly may be impracticable for large PSA units, owing to the weight of the rotating assembly. Also, when separating gas components which are highly inflammable or toxic, the rotary adsorbent bed assembly would need to be completely enclosed in a containment shroud to capture any leakage from large diameter rotary seals. Hence, PSA devices with stationary adsorbent beds will be preferred for larger scale systems, and for applications processing hazardous gases such as hydrogen.
In some of the above referenced prior art (e.g. Mattia, Boudet, and van Weenan), the rotary distributor valve would rotate continuously. Lywood (U.S. Pat. No. 4,758,253) and Kai et al (U.S. Pat. No. 5,256,174) have mentioned intermittent actuation of rotary multiport distributor valves for PSA systems, so that the distributor valve is stopped at a fully open position during each step of the cycle, and the distributor valve is then switched quickly to its next fully open position for the next step of the cycle.
It will be apparent that the multiport valves disclosed in the above cited inventions enable a simplification of PSA cycle switching logic, particularly those using multiple beds with pressure equalization steps, since the control functions of a multiplicity of two-way valves are consolidated into one or two multiport distributor valves. However, these prior art devices have limited utility except in small scale applications, owing to their lack of control flexibility. Since valve timing logic and port orifice sizing of the multiport valves are fixed rigidly in these prior art inventions, there is no provision for flow control to provide operational adjustment under changing feed conditions or during intervals of reduced product demand, or for performance optimization.
This inflexibility of control is most limiting for those of the cited prior art inventions which use multiport valves to exchange gas between a pair of beds, and across a pressure difference between that pair of beds. Such gas exchanges between pairs of beds arise in pressure equalization steps, in purge steps, and in product repressurization steps. For the PSA cycle to operate properly in a given application between given high and low pressures of the cycle, a correct amount of gas must be exchanged between a pair of beds in each such step, across the continuously changing pressure difference between that pair of beds during the step, and over the time interval of that step.
Especially in large industrial PSA systems, it is also necessary to avoid high velocity transients that could damage the adsorbent by excessive pressure gradients or fluidization. Such transients could occur as valve ports open at the beginning of an equalization or blowdown step. The internal geometry and orifice dimensions of a multiport distributor valve govern the amount of gas which can flow across a given pressure gradient over a given time interval. Once the internal orifice apertures of the rotary valves and piping connections have been fixed, the prior art PSA cycle using multiport valves could only operate correctly between given high and low pressures at one cycle frequency with a given feed composition, and would have no means for operational adjustment to optimize cycle performance.
Hence, prior art PSA devices with multiport valves would be unable to operate at much reduced cycle frequency during periods of reduced demand for purified product. It would be highly desirable to reduce cycle frequency when product demand is reduced, since lower frequency operation would be more efficient at lower flows, less stressful on the adsorbent and valve components, and less noisy in medical applications.
The ability to adjust operating frequency is also vital for applications where a product purity specification must be satisfied, while the highest attainable product recovery is desired from a feed mixture of given composition and flow rate and working between given higher and lower pressures. If the cycle frequency is too slow, the apparatus will release a relatively small exhaust flow at the lower pressure, resulting in high recovery of the light product at less than specified purity. If the cycle frequency is moderately too high, the apparatus will release a larger exhaust flow, achieving higher than desired purity and lower than desired recovery of the light product. If the cycle frequency is much too high, mass transfer effects may degrade performance to result in unsatisfactory light product purity as well as low recovery. Such applications arise for example in industrial hydrogen purification. In these applications, cycle frequency must be adjustable in order to achieve specified purity and simultaneously high recovery of the light product.
None of the cited prior art for pressure swing adsorption with multiport valves addresses the combined need for adjustable cycle frequency control and adjustable flow controls for gas exchanges between pairs of adsorbent beds. There is no flow control other than the pressure drop resistance of the conduits and the valve ports as they open and close. Hence, these devices as disclosed have the operational limitation that they cannot be operated at significantly varied conditions of cycle frequency and pressure.
It is well known that there is much scope for optimization of PSA cycles by adjusting the pressure intervals taken up by different steps. For example, Suh and Wankat (AIChE Journal 35, pages 523-526, 1989) have published computer simulation results showing the sensitivity to adjustment between the pressure intervals allocated to cocurrent and countercurrent blowdown. They showed that the optimum split between the pressure intervals for cocurrent and countercurrent blowdown is sensitive to the feed gas composition and the adsorbent selectivity. Product recovery performance is degraded by operation away from the optimum operating point.
The above cited PSA devices with multiport distributor valves lack any control means for making adjustments between the pressure intervals taken up by the different steps of the cycle. It would be very desirable to provide a control system capable of such adjustment while the PSA system is operating.
A further limitation of the prior art for PSA devices using multiport valves is the lack of control means to establish relatively smooth and constant flow over each step. Such control means could usefully alleviate the flow inrush at the beginning of each step when valve ports open across pressure differences, thus protecting the adsorbent bed and valve ports from transient flow velocities much in excess of the average flow during each step. Such control means could also minimize the time intervals of zero or much below average flow velocity during valve switching between steps, thus enhancing the productivity of the apparatus.
The pressure swing adsorption (PSA) process separates a feed gas containing a first component which is more readily adsorbed, and a second component which is less readily adsorbed, on an adsorbent material installed in adsorbent beds. The PSA apparatus has a number xe2x80x9cNxe2x80x9d of adsorbent beds operating in parallel, and phased 360xc2x0/N apart in operating sequence. Each adsorbent bed has a flow path through the adsorbent material, the flow path having a first end to which the more readily adsorbed fraction of the feed gas mixture is separated by the PSA process, and a second end to which the less readily adsorbed fraction of the feed gas mixture is separated. Cocurrent flow in the flow path is directed from the first to the second end of the flow path, and countercurrent flow is directed from the second to the first end of the flow path.
Pressure swing adsorption processes, including that of the present invention, include some or all of the following sequential and cyclically repeated steps for each of the adsorbent beds:
(A) feed step at the higher pressure of the cycle,
(B) one or more equalization steps for initial depressurization of the bed from the higher pressure to approach an equalization pressure, while gas withdrawn to depressurize the bed is supplied to another bed being pressurized in its step (F) toward the same equalization pressure,
(C) cocurrent blowdown of the bed to an intermediate pressure lower than the lowest equalization pressure but higher than the lower pressure,
(D) countercurrent blowdown of the bed to approach the lower pressure,
(E) purge step at substantially the lower pressure, with countercurrent flow of gas from step (C),
(F) equalization step(s) repressurizing the bed to approach an equalization pressure, with gas supplied to pressurize the bed being withdrawn from another bed undergoing step (B),
(G) repressurization of the bed to approach the higher pressure.
The present invention achieves pressurization and depressurization steps primarily by gas exchanges between the adsorbent beds. Steps entailing exchange of gas enriched in the second component between adsorbent beds will be described as light reflux steps. A predetermined logical sequence of the process steps will be established by rotary distributor valves, while flow regulation controls will enable satisfactory operation under varied process conditions and under varied cycle frequencies so that required product purity, recovery and output can be achieved by a simple control strategy.
The following terminology and definitions will be used hereunder for PSA devices using multiport distributor rotary valves. The first and second ends of the adsorbent beds are respectively connected in parallel to control valves which in the present invention include multiport distributor valves, a first distributor valve connected to the first ends of the adsorbent beds, and a second distributor valve connected to the second ends of the beds.
Each rotary valve has two relatively rotating ported valve elements, respectively the valve stator and rotor. The relative rotation of the valve elements sliding on a close contact sealing valve surface brings the ports of each element into sequential engagement. The valve surface is a surface of revolution, centered on the axis of revolution. The valve surface may be defined by flat discs, cones, circular cylinders, or other surface of revolution. The radial and axial position on the valve surface of a cooperating set of ports on the two valve elements must substantially coincide.
The adsorbent beds are connected to adsorbent bed ports on one of the valve elements, here described as the bed port element. External connections for feed supply, product delivery and exhaust discharge are made to function ports on the other valve element, here described as the function port element. Other function ports on the function port element will be provided for product reflux steps or for gas exchanges between pairs of adsorbent beds, e.g. for purge or pressure equalization steps.
The function ports have a critical role in defining the sequence and flow intervals for bed pressurization and blowdown steps. The present invention provides adjustable flow regulation controls, e.g. throttle orifices, on the conduits connecting pairs of function ports provided for gas exchanges between adsorbent beds. These flow controls may cooperate directly with either the bed port element or the function port element. In the example of pressure equalization steps, the flow controls must establish sufficient gas flow over the time interval of that pressure equalization step to achieve the desired pressure changes in the beds undergoing equalization, while avoiding excessively high transient gas flows that may damage the adsorbent. Adjustability of the flow controls is required to achieve a satisfactory pressure and flow regime, particularly when changing the PSA cycle frequency, working pressures, or feed gas composition or temperature.
The invention provides a process for separating first and second components of a feed gas mixture, the first component being more readily adsorbed under increase of pressure relative to the second component which is less readily adsorbed under increase of pressure over an adsorbent material, such that a gas mixture of the first and second components contacting the adsorbent material is relatively enriched in the first component at a lower pressure and is relatively enriched in the second component at a higher pressure when the pressure is cycled between the lower and higher pressure at a cyclic frequency of the process defining a cycle period; providing for the process a plurality of adsorbent beds of the adsorbent material with a number xe2x80x9cNxe2x80x9d of substantially similar adsorbent beds, with said adsorbent beds having first and second ends; and further providing for the process a first rotary distributor valve connected in parallel to the first ends of the adsorbent beds and a second rotary distributor valve connected in parallel to the second ends of the adsorbent beds, with flow controls cooperating with the first and second distributor valves; introducing the feed gas mixture at substantially the higher pressure to the first distributor valve; and rotating the first and second distributor valves so as to perform in each adsorbent bed the sequentially repeated steps within the cycle period of:
(A) supplying a flow of the feed gas mixture at the higher pressure through the first distributor valve to the first end of the adsorbent bed during a feed time interval, withdrawing gas enriched in the second component (light reflux gas) from the second end of the adsorbent bed, and delivering a portion of the gas enriched in the second component as a light product gas,
(B) withdrawing a flow of gas enriched in the second component (light reflux gas) from the second end of the adsorbent bed through the second distributor valve, so as to depressurize the adsorbent bed from the higher pressure toward an equalization pressure less than the higher pressure, while controlling the flow so that the pressure in the bed approaches the equalization pressure within an equalization time interval, and also controlling the flow so as to limit the peak flow velocity exiting the second end of the adsorbent bed in that time interval so as to avoid damaging the adsorbent,
(C) withdrawing a flow of gas enriched in the second component (light reflux gas) from the second end of the adsorbent bed through the second distributor valve, so as to depressurize the adsorbent bed from approximately the equalization pressure to an intermediate pressure less than the equalization pressure and greater than the lower pressure, while controlling the flow so that the pressure in the bed reaches approximately the intermediate pressure within a cocurrent blowdown time interval, and also controlling the flow so as to limit the peak flow velocity exiting the second end of the adsorbent bed in that time interval so as to avoid damaging the adsorbent,
(D) withdrawing a flow of gas enriched in the first component (countercurrent blowdown gas) from the first end of the adsorbent bed through the first distributor valve, so as to depressurize the adsorbent bed from approximately the intermediate pressure to approach the lower pressure, while controlling the flow so that the pressure in the bed approaches the lower pressure within a countercurrent blowdown time interval, and also controlling the flow so as to limit the peak flow velocity adjacent the first end of the adsorbent bed in that time interval so as to avoid damaging the adsorbent,
(E) supplying a flow of gas enriched in the second component (light reflux gas) from the second distributor valve to the second end of the adsorbent bed at substantially the lower pressure, while withdrawing gas enriched in the first component from the first end of the adsorbent bed and through the first distributor valve over a purge time interval, the flow of gas enriched in the second component from the second distributor valve being withdrawn from another of the adsorbent beds which is undergoing cocurrent blowdown step (C) of the process,
(F) supplying a flow of gas enriched in the second component (light reflux gas) from the second distributor valve to the bed, so as to repressurize the adsorbent bed from approximately the lower pressure to approach the equalization pressure, while controlling the flow so that the pressure in the bed approaches the equalization pressure within an equalization time interval, and also controlling the flow so as to limit the peak flow velocity entering the first end of the adsorbent bed in that time interval so as to avoid damaging the adsorbent, the flow of gas enriched in the second component from the second distributor valve being withdrawn from another of the adsorbent beds which is undergoing equalization step (B) of the process,
(G) supplying a flow of gas enriched in the second component (light reflux gas) from the second distributor valve to the bed, so as to repressurize the adsorbent bed from the equalization pressure to approach the higher pressure, while controlling the flow so that the pressure in the bed approaches the higher pressure within a repressurization time interval, and also controlling the flow so as to limit the peak flow velocity entering the second end of the adsorbent bed in that time interval so as to avoid damaging the adsorbent, the flow of gas enriched in the second component from the second distributor valve being withdrawn from another of the adsorbent beds which is undergoing feed step (A) of the process,
(H) cyclically repeating steps (A) to (G).
Steps (A) to (F) inclusive are conducted successively in the xe2x80x9cNxe2x80x9d adsorbent beds, in different phases separated by a fraction xe2x80x9c1/Nxe2x80x9d of the cycle period.
The invention provides an apparatus for separating the first and second components of the feed gas mixture, with:
(a) a number xe2x80x9cNxe2x80x9d of substantially similar adsorbent beds of the adsorbent material, with said adsorbent beds having first and second ends defining a flow path through the adsorbent material;
(b) light product delivery means to deliver a light product flow of gas enriched in the second component from the second ends of the adsorbent beds;
(c) a first rotary distributor valve connected in parallel to the first ends of the adsorbent beds; the first distributor valve having a stator and a rotor rotatable about an axis; the stator and rotor comprising a pair of relatively rotating valve elements, the valve elements being engaged in fluid sealing sliding contact in a valve surface, the valve surface being a surface of revolution coaxial to the axis, each of the valve elements having a plurality of ports to the valve surface and in sequential sliding registration with the ports in the valve surface of the other valve element through the relative rotation of the valve elements; one of said valve elements being a first bed port element having N first bed ports each communicating to the first end of one of the N adsorbent beds; and the other valve element being a first function port element having a plurality of first function ports including a feed port, a countercurrent blowdown port and a purge exhaust port; with the bed ports spaced apart by equal angular separation between adjacent ports; and with the first function ports and first bed ports at the same radial and axial position on the valve surface so that each first function port is opened in sequence to each of the N first bed ports by relative rotation of the valve elements;
(d) a second rotary distributor valve connected in parallel to the second ends of the adsorbent beds and cooperating with the first distributor valve; the second distributor valve having a stator and a rotor rotatable about an axis; the stator and rotor comprising a pair of relatively rotating valve elements, the valve elements being engaged in fluid sealing sliding contact in a valve surface, the valve surface being a surface of revolution coaxial to the axis, each of the valve elements having a plurality of ports to the valve surface and in sequential sliding registration with the ports in the valve surface of the other valve element through the relative rotation of the valve elements; one of said valve elements being a second bed port element having N second bed ports each communicating to the second end of one of the N adsorbent beds; and the other valve element being a second function port element having a plurality of second function ports including a plurality of light reflux withdrawal ports and light reflux return ports, with each light reflux return port communicating through the second function element to a light reflux withdrawal port; with the bed ports spaced apart by equal angular separation between adjacent ports; and with the function ports and bed ports at the same radial and axial position on the valve surface so that each function port is opened in sequence to each of the N bed ports by relative rotation of the valve elements;
e) drive means to establish rotation of the rotors, and hence relative rotation of the bed port elements and the function port elements, of the first and second distributor valves, with a phase relation between the rotation of the rotors and angular spacing of the function ports of the first and second distributor valves so as to establish for each adsorbent bed communicating to corresponding first and second bed ports the following sequential steps and cyclically repeated steps for those bed ports:
(i) the first bed port is open to the feed port, while light product gas is delivered by a light product delivery valve,
(ii) the second bed port is open to a light reflux withdrawal port,
(iii) the first bed port is open to the countercurrent blowdown port,
(iv) the first bed port is open to the purge exhaust port, while the second bed port is open to a light reflux return port;
(f) countercurrent blowdown flow control means cooperating with the first distributor valve; (g) light reflux flow control means cooperating with the second distributor valve; (h) feed supply means to introduce the feed gas mixture to the feed port of the first distributor valve at substantially the higher pressure; and
(i) exhaust means to remove gas enriched in the first component from the purge exhaust port of the first distributor valve.
The flow control means cooperating with the first and second distributor valves (for respectively countercurrent blowdown and light reflux steps) may be provided as continuously adjustable orifices (e.g. throttle valves), or as discretely adjustable orifices with selector valves to switch between discrete settings. The light reflux flow control means may be provided as adjustable orifices within the rotor of the second distributor valve, or as adjustable orifices interposed between the second end of each of the adsorbent beds and the second distributor valve.